(1) Field of the Invention
This invention generally relates to a device for thermal detection of seawater. More particularly, the invention relates to a device for thermal detection of seawater from within an unmanned underwater vehicle, thereby determining a position of a thermal boundary with respect to the vehicle.
(2) Description of the Prior Art
The current art for presetting underwater vehicles in search of a target is complicated by the presence of thermal layers beneath the water surface. A thermal layer can serve as an acoustic barrier by refracting transmitted sound waves (sonar) thereby isolating the target from the pursuing vehicle. The vehicle operator will thus attempt to position the vehicle by presetting the vehicle at the same depth as the submarine or at least on the same side of the thermal layer as its target to optimize its chance of achieving acoustic detection.
FIG. 1 illustrates the known characteristics of a thermal layer 10 in an underwater environment. Specifically, a vehicle 12 may be on a first side 11 of the thermal layer 10, while the target 14 is on an opposite side 11′ of the same thermal layer 10. This figure clearly shows how a target 14 can avoid detection by a sonar signal 13 of vehicle 12 merely by positioning itself on the opposite side of thermal layer 10 from the vehicle 12. Thus, a problem exists in the art whereby it is necessary to quickly and effectively determine the location of the thermal layer 10 in order to position the vehicle, as at 12′, on that side 11′ of the thermal layer 10 which will correspond to the target 14. The target 14 will then be detected as shown in the lower half of FIG. 1 by sonar signal 13′.
The current technique of presetting many underwater vehicles such as the Mk 46 torpedo requires the operator to select (preset) a depth that the vehicle will use to initially locate a submerged target. This initial preset search depth is chosen based on knowledge of three variables: (a) water depth, thermal layer depth, and (c) target depth.
The water depth provides a lower boundary below which the vehicle cannot pass. By using ocean charts and maps, the operator can identify the average local water depth to a reasonable degree of accuracy.
The thermal layer depth is a function of ocean currents, the ambient weather conditions, and time. Additionally, temperature/depth profiles are only accurate when and where the data is taken. Consequently, the locations of any thermal layers are only known approximately when they are known at all.
The depth at which the target is located is rarely known to any significant degree of precision unless it happens to be at the surface. If it were known, the operator would always select the vehicle's initial search depth to match the target depth since this would automatically place both vehicles on the same side of the layer.
Since the thermal layer depth and the target depth are generally not well known by the operator, the vehicle's initial search depth is usually preset with an educated guess. If the vehicle and target are on opposite sides of the thermal layer such as the situation occurring in FIG. 1, the mission may only succeed if the vehicle 32 and target 34 pass relatively close to each other since the successful acquisition range may be very short under these circumstances. To complicate the mission, a target 34 can evade the pursuing vehicle by changing depth to place itself on the opposite side of the thermal barrier 30.
The following patents, for example, disclose various types of depth sensors, but do not disclose a thermal detection system within a vehicle as does the present invention which permits a detection of underwater thermal boundary layers.                U.S. Pat. No. 3,802,365 to Reeser;        U.S. Pat. No. 3,882,808 to Francois et al.;        U.S. Pat. No. 4,239,012 to Kowalyshyn et al.;        U.S. Pat. No. 4,323,025 to Fisher et al.; and        U.S. Pat. No. 5,819,676 to Cwalina.        
Specifically, Reeser disclose a depth responsive override system for correcting torpedo command signals directing the torpedo beyond a pre-selected depth, including a depth sensor and an adjustable depth signal source combined in a first differential amplifier to produce a difference signal indicative of the difference of the actual depth minus the pre-selected depth. A function generator is connected to receive the depth difference signal and the torpedo command signal for producing an override output signal during the times when the pitch command signal is greater than the difference signal. A second differential amplifier is connected to receive the pitch command signal and the function generator output signal for producing an output signal to control the torpedo elevators.
The patent to Francois et al. discloses a method of effecting control and guidance of an anti-submarine torpedo of the type having a high yield warhead. The torpedo is launched from a hunter submarine against a submerged target submarine. The torpedo warhead when exploded beneath the surface of the water has explosive properties such that the maximum distance at which a predetermined damage inflicting effect occurs increases in a predetermined manner in accordance with the depth at which the warhead is exploded. This method includes the steps of placing the hunter submarine at a torpedo launching depth and launching the torpedo. The hunter submarine is then maintained at the depth or above until the warhead is exploded. The torpedo is then guided outwardly from the hunter submarine along a first substantially horizontal course. The torpedo is next guided downwardly and outwardly through the safe-to-detonate volume along a second slant dive course at a fixed dive angle after the torpedo travels across said surface of revolution and into the safe-to-detonate volume. A third horizontal course is provided at a desired explosion depth. The warhead detonates at a desired distance from courses within said safe-to-detonate volume.
Kowalyshyn et al. discloses a homing torpedo control apparatus in an echo-ranging torpedo wherein spurious and tru-target echo signals may be received in the listening periods between repetitive search-pulse transmission instants, in combination: a receiver operative to convert received echo signals to steering command signals having characteristics corresponding to echo-source direction, said receiver including a target-recognition circuit and a gating relay which operates, with inherent delay, in response to recognition of each tru-target echo signal; a steering control circuit, including steering relay and a switch means, adapted to place said steering relay switch means in a condition corresponding to echo-source direction as derived by said steering control signals; means applying said steering command signals to said steering control circuit; means controlled by operation of said gating relay, in response to each reception and recognition of a tru-target echo signal, to render said steering control circuit operative to respond to steering command signals stemming from subsequent echo signals during a predetermined interval, of the order of a few listening periods, following each said reception and recognition of a tru-target echo signal, and a steering apparatus responsive, when rendered effective, to said steering relay and switch means condition; said gating relay, when operated controlling said steering apparatus to render it effective to respond to said steering relay and switch means condition.
Fisher et al. disclose a torpedo steering control system including means for providing a target search phase of torpedo operation wherein said torpedo is controlled to change depth between predetermined search floor and search ceiling depths and simultaneously controlled to circle in azimuth, whereby to execute helical search action; means for switching, in response to target acquisition at any time during a search phase of torpedo operation, to a target pursuit phase of torpedo operation; and means for switching, in the event of and in response to target loss continuing for a predetermined period in a target pursuit phase, to a modified search phase of torpedo operation wherein said torpedo is initially controlled to execute circle search action while maintaining depth position at substantially that at which target loss occurred, for a predetermined period accommodating at least a complete azimuth circling turn, then controlled to revert to helical search action.
Cwalina discloses a search angle selection system to determine acoustic homing beam offset angles to be used by a torpedo from a group of target depth conditions in response to given environmental, tactical, target and vehicle information. The system optimally bounds the region that is to be insonified. The system determines the search angle which best insonifies the depth band, that is, the region between the upper depth bound and the lower depth bound, for each search depth, accounting for the vehicle's attack angle, including search depths which are not in the depth band itself. For each search depth, the system determines the relative depth separation of the search depth from the each of the bounds, and based on this separation an aimpoint which projects from a reference plane through the torpedo is chosen at the depth of each bound. The aimpoint is selected from a table of empirically-determined values. The system modifies the aimpoint when strong negative gradients in the sound velocity profile are present in the ocean environment, and also in the case of strongly conducted rays. A reference insomnification beam axis angle is iteratively determined for reach search depth with the axis causing a raypoint which intersects along the respective bound. The pair of reference beam axes whose ray paths intersect the upper and lower bound at the aimpoint for each search depth are averaged to provide the optimal homing beam angle for that search depth.
In view of the prior art, the present inventors have discovered that if the vehicle can internally identify the thermal layer, it can then change depth and actually use the layer to its own advantage by focusing the sonar signals toward the target. It should be understood that the present invention would in fact enhance the functionality of the above patents by providing a thermal sensor within the vehicle of choice for sampling seawater and determining the presence of a thermal layer when encountered. Additionally, the system is such that the depth of the vehicle is alterable in response to the detection of the thermal layer, thus enhancing the ability of the vehicle to locate a subject target.